A standard pressure-balancing valve as described in U.S. Pat. No. 4,022,242 of Turecek or European 0,322,597 of Hochstrasser has a housing formed with a pair of inlet ports, a pair of coaxial tubes having outer ends seated in the housing at the ports and opposite confronting, axially engaged, and axially closed inner ends. The tubes are each formed with a radially throughgoing aperture. A flexible annular membrane has an inner periphery sealingly engaged around and the tubes and an outer periphery seated in the housing and defines a respective outlet compartment around each of the tubes. A pair of like sleeves coaxially surrounding the tubes each have an inner end fixed to the membrane offset from the center and from the outer periphery and an outer end. Formations on the outer ends of the sleeves and on the tubes form annular valve openings of flow cross sections dependent on an axial position of the sleeves relative to the tubes. The compartments communicate via the respective valve openings and apertures with the respective inlets so that, when pressure at one of the ports exceeds pressure at the other port, the sleeves will be moved axially by deforming the membrane to increase the flow cross section of the valve associated with the other port and decrease the flow cross section of the valve associated with the one port.
There is a sliding-seal joint between either the sleeves or the inner periphery of the membrane and the stationary tubes. Thus as the membrane flexes to equalize pressure in the two outlet compartments, some part slides on the tubes. This sliding must of course be allowed with minimal friction so that the valve can be as responsive as possible, and it must not allow any leakage across the joint. Thus O-rings are usually used in combination with a lubricated joint.
With time, however, the sliding action becomes stiffer and stiffer. The lubricant is washed away and the seals harden. In addition lime deposits on the surfaces make them rougher. Thus eventually the ability of the membrane to move is largely lost, along with the pressure-balancing function of the valve. The only solution is replacement of the membrane and core of the valve.